Alterations in insulin signalling pathway induced by prolonged insulin treatment of 3T3-L1 adipocytes

Insulin-induced glucose transport stimulation, which results from the translocation of glucose transporter 4 (GLUT 4)-containing vesicles, is completely blocked after prolonged insulin treatment of 3T3-L1 adipocytes. Since GLUT 4 expression was reduced by only 30%, we looked at the insulin signaling pathway in this insulin-resistant model. Insulin-induced tyrosine phosphorylation of the major insulin receptor substrate IRS 1 was reduced by 50 +/- 7%, while its expression was decreased by 70 +/- 4%. When cells were treated with worthmannin (a PI3-kinase inhibitor) together with insulin, the expression of IRS 1 diminished to a much lower extent. Associated with the decrease in IRS 1 expression and phosphorylation, the activation by insulin of anti-phosphotyrosine immunoprecipitable PI3-kinase activity and of p44mapk activities was altered. However, the expression of these proteins was normal and p44mapk activity remained responsive to the tumour promoter TPA. Those results indicate that prolonged insulin treatment of 3T3-L1 adipocytes induces an insulin-resistant state with a reduced ability of insulin to stimulate the PI3-kinase and the MAP-kinases and a blockade of glucose transporter translocation.